Measurement of nematic liquid crystal splay elastic constants by disclination line motion in high pretilt, negative dielectric anisotropy pi-cellsHalf-leaky guided wave determination of azimuthal anchoring energy and twist elastic constant of a homogeneously aligned nematic liquid crystalWe present a fast and accurate method for measurement of the twist elastic constant of nematic liquid crystals using pi-cell devices. We observe that there are two bifurcations during the transition from the bend state to the planar 180°twisted state, each of which leads to a voltage threshold. The Fréedericksz threshold voltage due to a tilt bifurcation occurs at a lower voltage and is "softened" by the required surface pretilt in the device. The higher of the two thresholds is due to a bifurcation between left-and right-handed 180°twisted states and we term this the T to V state threshold. This voltage threshold is independent of device thickness, and is not softened by the pretilt. We show that the T to V state threshold is highly sensitive to changes in the twist elastic constant, K 22 , and use this threshold to evaluate the magnitude of K 22 for a variety of materials. These results show very good agreement with those obtained using the standard magnetic Fréedericksz twist threshold experiment. This method does not require a complex experimental setup and has an accuracy of better than ±8%.
A technique for measuring the difference between the splay and bend flexoelectric coefficients in a nematic liquid crystal is demonstrated. The method uses the flexoelectric-optic effect, but instead of a uniform lying helix structure, a TN cell geometry with an in-plane electric field is used. This has the advantage of avoiding difficulties associated with aligning the helix and can be used to measure achiral materials. The effects due to ionic screening are also taken into account.
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